The engraving was done in 1822, the painting in 1817. The Wikipedia caption for this image states, "An engraving based on Sir Thomas Lawrence's painting of Charlotte, which she sat for in her final days." It or one like it is at the National Portrait Gallery, London.
This is not just another portrait of a woman in classical dress - Charlotte is with child in this portrait. Maternity dress was rarely shown since around 1600. For example, no maternity portraits are around for Queen Victoria who bore nine children.